EMM cause (LTE NAS)3GPP TS 24.301 §9.9.3.9 and Annex A
What it means
Implicitly detached: the network had already detached the phone (e.g. after its timers expired). The phone has to attach again.
Where you see it
In LTE NAS Attach reject, Tracking area update reject, Service reject or Detach request.
What to check
Normal after long inactivity; investigate only if it repeats right after a successful attach.
Look at the S6a (MME↔HSS) and S11 messages just before the reject: they usually show the real trigger.
Got a trace with EMM cause #10 Implicitly detached? Drop it in the analyzer: it finds the step that really failed first, which node sent the code and what to ask the other side. Captures, test-system PDFs and vendor exports. 5 free analyses a month, no sign-up.