Spotting an air leak with the naked eye is next to impossible, and while you have sealant in your tyres this is designed to stick to the reverse side of the tread to prevent punctures rather than where the tyre is seated against the rim. One thing you can do is fill a spray bottle with a strong mix of washing up liquid and water, then spray this around the rim of the inflated tyre, as any leaks should create bubbles, and this approach would also work on the valve too. If there is no puncture and the valve is working as it should then the most likely issue is the the seating between the wheel and the tyre. Particularly on older wheels, even a small distortion in the shape of the rim caused by a bump or pothole can cause a slow leak, while an imperfect surface caused by corrosion or failing paint can also cause a leak. We would suggest having the wheel inspected by an alloy refurbishment specialist who can check the condition and trueness of the wheel.