My switch 13 (same panel) is for the tricolor light and has an appropriate symbol for it. Yours looks more like a heater symbol, so possibly there was/is a heater of some kind (diesel, propane, electric stove ignition, etc.)?
"frei" means "free", so if you connect these to anything, you should print a label and stick it over it to tell what they're for
My switch 16 is indeed free now (it was previously used for a bodge-fix to switch off the chartplotter, which had a non-working power button).
If you really want to know for certain, follow the wire!