I’ve been losing 1-2psi every day whether I’m riding or not. I’ve taken it to a garage. They checked the tyre and the valve, but could not find any leaks and suggested I use tyre sealant slime. I have done that now, but the situation remains unchanged. I’ve also replaced the valve cap and used two different pressure gauges.
Slime is nasty shit and a lot of places will charge extra to clean that crap out when the tyre is replaced. The best way to find the leak is too over pressure the tyre and put in a trough of water and look for air bubbles. Or use really soapy water.
How old is the bike? It could be a rim leak if the rim is corroded on the inside. They are harder to find. They can clean the rim to prevent it happening in future. That requires taking the tyre off.
I bought a small kiddies inflatable paddling pool that I slip under the wheel and then fill with water. Saves having to remove the wheel. For one particular leak it worked really well but generally a spray bottle with a very very weak solution of washing up liquid works better. Just spray on and wait.
Sealant slime is Bad News. I learned many years ago that fitters hate it with a passion and as Kevsta says they sometimes charge extra to clean it up.
A spray bottle with soapy water is a Good Idea, you need patience though if it’s a really slow leak…
I’ve bought a new wheel in the past with good tyre on it for less than the cost of a new tyre fitted - that might be worth a go. In my case I was also able to sell the old wheel. Of course, depending on your bike this might not be an economical solution