diff --git a/app/deposits/service.py b/app/deposits/service.py
index d515d91..9c3cd34 100644
--- a/app/deposits/service.py
+++ b/app/deposits/service.py
@@ -54,3 +54,49 @@ async def credit_confirmed_utxos(session: AsyncSession, user_id: int, entries: l
         broadcaster.publish()  # nudges this user's dashboard to refetch its balance instantly
 
     return newly_credited
+
+
+_EXTERNAL_SPEND_SENTINEL = "external-spend"
+
+
+async def detect_external_spends(session: AsyncSession, user_id: int, entries: list[dict]) -> int:
+    """Mirror of credit_confirmed_utxos: catches a UTXO leaving the address
+    through a transaction this platform never built (e.g. someone spending it
+    directly with the raw privkey, bypassing /withdrawals entirely).
+
+    Everything the platform itself spends (bets, withdrawals, payouts) sets
+    spent_txid at broadcast time, before the tx ever reaches the chain — so by
+    the time an Electrum refresh runs, an outpoint still marked unspent in our
+    own DB that Electrum no longer reports as unspent was never on our radar.
+    entries is this address's current `listunspent`; anything in our unspent
+    set but missing from it left the address some other way. Returns the
+    number of UTXOs newly marked spent.
+    """
+    current_keys = {(entry["tx_hash"], entry["tx_pos"]) for entry in entries}
+
+    unspent_rows = (
+        await session.scalars(
+            select(UtxoEvent).where(UtxoEvent.user_id == user_id, UtxoEvent.spent_txid.is_(None))
+        )
+    ).all()
+
+    newly_spent = 0
+    for row in unspent_rows:
+        if (row.txid, row.vout) in current_keys:
+            continue
+        row.spent_txid = _EXTERNAL_SPEND_SENTINEL
+        await write_audit_log(
+            session,
+            "utxo_spent_externally",
+            {"txid": row.txid, "vout": row.vout, "amount_sats": row.amount_sats},
+            user_id=user_id,
+        )
+        newly_spent += 1
+
+    if newly_spent:
+        await session.flush()
+        await recompute_balance(session, user_id)
+        await session.commit()
+        broadcaster.publish()
+
+    return newly_spent
diff --git a/app/electrum/listener.py b/app/electrum/listener.py
index 6514667..42c7c43 100644
--- a/app/electrum/listener.py
+++ b/app/electrum/listener.py
@@ -6,7 +6,7 @@ from sqlalchemy import select
 from sqlalchemy.ext.asyncio import async_sessionmaker
 
 from app.db.models import User
-from app.deposits.service import credit_confirmed_utxos
+from app.deposits.service import credit_confirmed_utxos, detect_external_spends
 from app.electrum.client import ElectrumClient, ElectrumEndpoint
 from app.electrum.scripthash import address_to_scripthash
 from app.rounds.events import broadcaster
@@ -207,5 +207,8 @@ class ElectrumListener:
         entries = await self.client.listunspent(scripthash)
         async with self._session_factory() as session:
             credited = await credit_confirmed_utxos(session, user_id, entries)
+            spent_externally = await detect_external_spends(session, user_id, entries)
         if credited:
             logger.info("credited %s new UTXO(s) for user_id=%s", credited, user_id)
+        if spent_externally:
+            logger.warning("%s UTXO(s) spent outside the platform for user_id=%s", spent_externally, user_id)
diff --git a/tests/unit/test_deposits.py b/tests/unit/test_deposits.py
index 8571dd3..299ec9c 100644
--- a/tests/unit/test_deposits.py
+++ b/tests/unit/test_deposits.py
@@ -1,9 +1,10 @@
 import pytest
+from sqlalchemy import select
 from sqlalchemy.ext.asyncio import async_sessionmaker, create_async_engine
 
 from app.db.base import Base
-from app.db.models import User
-from app.deposits.service import credit_confirmed_utxos
+from app.db.models import AuditLog, User, UtxoEvent
+from app.deposits.service import credit_confirmed_utxos, detect_external_spends
 
 
 @pytest.fixture
@@ -52,3 +53,33 @@ async def test_idempotent_on_repeated_notification(session_factory, user_id):
         assert first == 1
         assert second == 0
         assert user.cached_balance_sats == 7_000_000
+
+
+async def test_external_spend_marks_utxo_spent_and_corrects_balance(session_factory, user_id):
+    entries = [{"tx_hash": "dd" * 32, "tx_pos": 0, "height": 100, "value": 20_000_000}]
+    async with session_factory() as session:
+        await credit_confirmed_utxos(session, user_id, entries)
+
+    async with session_factory() as session:
+        spent = await detect_external_spends(session, user_id, [])
+        assert spent == 1
+        user = await session.get(User, user_id)
+        assert user.cached_balance_sats == 0
+
+        utxo = (await session.scalars(select(UtxoEvent).where(UtxoEvent.user_id == user_id))).one()
+        assert utxo.spent_txid == "external-spend"
+
+        audit_events = (await session.scalars(select(AuditLog))).all()
+        assert any(e.event_type == "utxo_spent_externally" for e in audit_events)
+
+
+async def test_no_spend_detected_when_utxo_still_unspent(session_factory, user_id):
+    entries = [{"tx_hash": "ee" * 32, "tx_pos": 0, "height": 100, "value": 3_000_000}]
+    async with session_factory() as session:
+        await credit_confirmed_utxos(session, user_id, entries)
+
+    async with session_factory() as session:
+        spent = await detect_external_spends(session, user_id, entries)
+        assert spent == 0
+        user = await session.get(User, user_id)
+        assert user.cached_balance_sats == 3_000_000