Are you drilling horizontaly or verticaly to the outlet, usualy this
is the way wires are run, but not 100% of time. Especialy in old
remodels. Are they in conduit, if yes go slow, with resistance stop. TO
BE 100% SAFE knock a hole, and re plaster. .
Zircon a relativly cheap Stud finder incorporate this feature ,
My Zircon does, but I wouldnt trust diagnosing led patterns, studs
are tough enough to find . Drill in , look with a light , and drill.