short somewhere?

Growing up a poor boy back in the day I always used a test light as mentioned above. Didn’t have a meter. It was a dash light sized bulb with a couple of feet of wire and alligator clips. I still do it today. Digital meters can fool you sometimes if you are not used to them.

I would take off a battery clamp, hook one lead of the lamp to the battery cable and the other to the empty battery terminal. If the lamp lights up, you have a current draw. If the lamp lit up, I would touch the battery cable back to the battery for a second or two. This would charge up the radio or delayed dome light or what ever else might draw current temporarily. If the light went out after that, everything was ok. If not, more troubleshooting.

Newer cars with a lot of electronics may take a substantial current to charge everything up in initially.

As I recall, some aftermarket radios and alternators with built in regulators will draw a small current all the time, on the order of micro amps, but this will not light a dash light sized bulb or drain your battery in less than about a month.

This is just one technique for troubleshooting, may not work in every situation.