I think it's a good idea to try to keep a weight record on your chins, it can be hard to notice weight lose just by looking and feeling them until they have lost a good bit. You don't have to weigh them daily, at least once a month is good so long as they are seemly healthy. Weight loss can often be the first sign something is wrong, before you even notice any behavior changes. As said though a small weight loss is not something to be concerned with unless it continues to go down. If you notice a major weight change from weigh in to weigh in it's best to just weigh again a few times over the week to just double check it's really a weight lose and not just timing. The bladder and stomach can each hold about 5-10 grams each, so having just ate, drank, or went potty or not can cause weight fluctuation (as much as 10-20g if the bladder and stomach are both full at the same time) so it's best to try to do it at the same time of day each time.
Don't feel too bad if you aren't good at keeping records, I sometimes go months between weighing my guys because I keep forgetting about it. It's a good idea to have an idea of what your chins should weigh when healthy though. A digital scale that measures in grams is best since chins weigh so little grams are more precise.
