What causes tire pressure loss?

I found my front Driver's tire on my 1998 SL1 today with almost no pressure in it. It wasn't flat but the tire gauge couldn't read anything, I think it starts at 10. I filled it up to max, drove for a few hours then checked when I got back, it was 2 lower than max. Just checked again after a few hours, another 1 lower total 3 from when I maxed it. I have also checked the tire 3 times in three different locations, I don't see anything that could cause a leak, no puncture marks. I think this is one of two original tires, I have all my maintenance/service records, only once did I have 2 tires changed when I had a flat. How long until tires wear out?
Thanks.

Eric

You probably have a very small leak that isn't visible. Probably the valve stem.

Just take it to a tire shop and they will fix it up. I had something similar on my tire a month or so ago.

If you want to find the leak, get some dish soap and make a bubble solution. Soak it down good and look for bubbles, a spray bottle helps. It may take a while to find it sometimes. One thing to keep in mind, fluctuations between 1 to 3 lbs on any given day are not unusual because of heat. Driving heats the tires/air in the tires and causes the pressure to go up. On a cold day or before driving, the pressure would be lower.

An original tire from 1998 is likely quite worn out unless it has really low miles, or if not it's probably dry-rotted causing a leak.