BT MemoryDecay - use FActorPerceptionInfo::GetLastStimulusLocation(&OutAge)

TL;DR: Unreal Engine's UAIPerceptionComponent already tracks per-actor last-known stimulus location with age. No need to roll your own decay timer.

Pattern (in your AIController or a small custom service)

UAIPerceptionComponent* Perception = GetAIPerceptionComponent();
if (!Perception) return;

// Pull the last-known info for a specific actor (e.g. the threat you're tracking).
if (const FActorPerceptionInfo* Info = Perception->GetActorInfo(*ThreatActor))
{
    float Age = 0.0f;
    FVector LastKnown = Info->GetLastStimulusLocation(&Age);

    // Apply your project's memory window.
    if (Age < MemoryWindowSec)
    {
        Blackboard->SetValueAsVector("LastKnownThreatLoc", LastKnown);
        Blackboard->SetValueAsFloat ("MemoryAge",          Age);
        Blackboard->SetValueAsBool  ("bHasMemory",         true);
    }
    else
    {
        // Memory expired.
        Blackboard->SetValueAsBool("bHasMemory", false);
    }
}

That's it. Perception's age tracking handles refresh-on-stimulus + decay automatically.

How to integrate as a BT service

Wrap the snippet above in a UBTService_BlueprintBase subclass (Blueprint-only - no C++ needed):

1. Create a BP service. Override Receive Tick.
2. Inputs: ThreatActor (Object BB key) + MemoryWindowSec (float).
3. Inputs to write: LastKnownThreatLoc (Vector BB key) + MemoryAge (Float BB key) + bHasMemory (Bool BB key).
4. Tick body: get perception component → GetActorInfo → GetLastStimulusLocation(&Age) → branch on Age vs window → write BB keys.

Five nodes total. No C++ required.

Why we didn't ship LCM_BTService_MemoryDecay

The stock FActorPerceptionInfo::GetLastStimulusLocation(&OutAge) provides everything our planned MemoryDecay service would - location + age tracked per sense, per actor, automatically refreshed on stimulus and aged each frame.

Rolling our own decay timer would:

• Duplicate the perception component's bookkeeping
• Miss the per-sense granularity (perception tracks separately for Sight / Hearing / Damage / etc.)
• Force buyers to maintain two parallel "last seen" caches

The LCM Nav3D StateTree variant FLcmSTEval_Memory. Same conclusion on both sides.

Related LCM Nav3D-unique services

If you want squad-level memory (where multiple allies' perceived-actor lists are aggregated), use LCM_BTService_SquadSnapshot - that's a genuinely LCM Nav3D-additive surface.