Frequency changes
Urinating noticeably more often can point to kidney or thyroid issues, or diabetes. Straining, or making frequent trips with little result, is more urgent, particularly in male cats, where it can indicate a blockage.
Consistency and appearance
Diarrhoea lasting more than a day or two, or any blood in urine or stool, is worth a call rather than a wait. A single soft stool after a diet change is usually nothing. A pattern is a different matter.
Location changes
A cat suddenly avoiding the litter box, or going just outside it, is often communicating something, discomfort while eliminating, a box that feels unsafe, or a territorial issue with another pet. It's rarely simple stubbornness, and it's worth investigating rather than only addressing as a behaviour problem.
Why this is worth writing down, not just noticing
None of these changes are dramatic in the moment, which is exactly why they're so easy to half-notice and then forget. A quick note in Halo's daily log turns "I think that's been happening more lately" into an actual answer at the next vet visit.