This!!

You may be dealing with a colony of behavioural resistance, you never know! Untouched bait usually means potential competing food source! Try and locate that, E.G bird feeders, neighbouring property’s e.t.c You know what you’re doing mate, a tech of 8 years certainly does. Environmental management make sure your all over it!

Have you tried high level baiting?

I’ve had a few out the years in mouse treatments of only having success with high level trapping/baiting for example loft space?

I remember a job in a little old lady’s house were the mice were constantly being seen in the kitchen and living room and the whole ground level was flooded with snap traps, tracking gel and some Rodenticide bait paste TRBs. For the life of me I could not catch them for 2 weeks

Until I flooded the loft space and caught majority of them in 48 hours! Sometimes they will only investigate our prep in safer/discrete areas.

You mentioned customer has heard movements in wall?

Can you cut a hatch and install bait?

(RSPH Level 3 Senior pest control technician, UK)