Coolant Tank (Small Cap) - Corolla

[SHL]

I was checking the coolant level on my Corolla and mistakenly hit this small cap that sits just below the main tank cap. It fell somewhere into the abyss and for the life of me I cannot find it as it's so small. What's the purpose of this tiny cap and is it safe to drive without it?

 

[TheChamp]

Probably some unused overflow feature.