Could they be on holiday, or at work, or at the shops, or visiting Auntie So and So? Anything that you think might satisfy her for the moment, really.
I used to use 'at the shops' and 'visiting Auntie X' when my FIL started asking where MIL was. She'd been dead 10 years - the truth would upset him terribly and in any case he would only forget very quickly and ask again.