Sometimes one speaks of a senior moment, as defined here
a temporary mental lapse (humorously attributed to the gradual loss of one's mental faculties as one grows older).
Example: "Oops, I think I forgot to lock the back door. Senior moment!"
Or, more to the OP: "I know I forgot something. My slippers? the Swiss army knife? My hair dryer? Too many senior moments lately!"
This phrase does not yet need a euphemism, but better not to use in the company of someone who really does have dementia, or their caregiver.