Very Tiny Battery in iMac 2010

Have issue with HD so opened up iMac to remove it and familiarise myself with process for replacement, have yet to get replacement HD. Somewhat dusty, so took the opportunity to get the hairdryer on it, cool setting, which was done whilst the machine was on its back. upon putting it upright, I heard a noise, something metal moving around, so tipped the machine upside down and it fell out. Having found what I think is the culprit, all screws had been accounted for, and it was a tiny battery; not a coin size that Google search says is for the PRAM / CMOS, but far smaller, 0.4mm diameter x 0.3mm high.

I cannot find any reference to such a size battery for the iMac,; and I do not really want to look for an empty battery housing on the other side of the MB with all that entails, so wondering if it was a rogue one that had fallen in during build. The work was completed on my dining table and there is no reason why such a battery would be coincidentally there.

The iMac can still power up, although cannot find the HD, being the original issue, but I can still access macOS Recovery systems, which is not showing the HD (Macintosh HD), same as before.

Hopefully I have covered everything.

Any suggestions as to what this battery is for?