She hears bells ringing and thinks I have set alarms, thinks I have put stickers on plug sockets and is sure someone is coming into her flat when she goes out. This is very distressing for her. Her health otherwise is so good and no sign of memory problems. She is very sharp and knows what is going on apart from these episodes.
1st suspect as others already said is UTI.
Or other infections, low sodium, kidney function, stroke - many things the Doctor will want to rule out.
Is there a regular time of day this happens? End of day fatigue?
Sundowning, or sundown syndrome, is a neurological phenomenon associated with increased confusion. Faded light, low light, and increased shadows may contribute to confusion and fear. Or upset in their circadian rhythms.
Don't jump to sundowners conclusion, but discuss with the Doc too.
Make sure you take a list of any medications she is currently taking, including over-the-counter as well as prescribed ones.
Big hugs to you both.