It is very unlikely that you have any anti-freeze in your fin-tube baseboard and even less likely that you would smell it unless you have an active leak. If you had anti-freeze it would most likely be propylene glycol which is generally non-toxic even food grade- and if by chance you had ethylene glycol (common in automobile anti-freeze and commercial boiler systems) in your baseboard fin-tube, you would have to drink it to be get sick and yes it would be fatal for a cat or dog.

I would suggest that the smell comes when fin his hot and therefore is coming from the outside of the fin. Something on or near the fin is off-gassing, say the carpet below.