I suppose you need to consider what the plan is if you do catch some incriminating footage. I'm not sure the police would accept it as evidence, and you may not want to confront the individual.
You could also consider moving some of the higher value stuff (either monetary or sentimental) out of the house, to take away any incentive, with or without their knowledge. Sounds like the poor old dears may not even notice it's missing, and if they did you could explain why. (I'm thinking smaller stuff like jewellery or cash, which is likely to be the targetted stuff anyway)
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