My husband found this little silicone thing in the sink while doing the dishes. I have no idea where it comes from, but I swear I've seen it before. Ideas?

Conclusion: solving the mystery and prevent losses
The discovery of the small silicone object in our sink proved to be a valuable lesson in kitchen safety and equipment maintenance. The identification of a float valve gasket in our pressure gauge not only solved the mystery, but also stressed the importance of regular checks and good cleaning practices. To avoid future losses, we plan to be more vigilant when cleaning up and consider setting up a space dedicated to small parts of appliances. This experience has reinforced the importance of each component, however small, in ensuring the safe and effective operation of our cooking utensils.