It may not be something to worry about! As a plant gets older, so do the leaves. Every now and again, your plant will drop one of the oldest leaves to sustain the newer growth. The yellowing is usually very even and always one of the oldest leaves, especially if the leaf is over a year old for certain types of plants!
Below are some examples just looking amongst most of my plants. They're still happy and healthy, just making way for new growth 🥰