Duty bound, yes if they pay or arrange it on say the last maint visit. As above you are protecting yourself against a claim.
The customer owns the system but the programming could be deemed intellectual rights of the incumbent.
I would want to know about any tampering of a system I had under contract. Sounds like you are on a winner if they are calling you out to crash and reset imo, but I wouldn't see them as long term customers.
I've had them for years, just bought a new one. I've been lucky enough not to need it in anger, but I am aware a large percentage of the time I'd be crawling through the wreckage to eat the sd card.