Water heater T&P valve very slow drip

I have AO smith water heater installed 4 years ago. No expansion tank and not a closed loop system. Everything was fine until recently. I noticed the T&P drips very very slowly, maybe a drop or two when the water heater runs. I've also just added an expansion tank and T&P valve still drips very very slowly. We have never "tested" the T&P valve in these 4 years. Since it's been working until recently, could this just be a bad T&P valve?

Thanks