A doubt 5 watts of heat is enough to even get hot or explode the battery unless is was poorly vented such as in a sealed box.
What happens is the sulphuric acid electrolyte (H2SO4) liberates Hydrogen easiest from excess energy wasted and if there is a spark with H2 in a container it can be dangerous as 4% H2 plus any amount of oxygen is an explosive condition with a tiny spark. But with vented H2 it dissipates in the atmosphere quickly and rises (lightest gas) out of reach, so is not a problem. But in large car batteries under a hood with enclosed gas and an spark from a loose jumper cable, it could detonate. (big boom)
Most likely the battery has now dried its electrolyte and if after a long time the battery is worn out.
FYI Sealed gel cell batteries MUST have a vent . In external cases it must have an added Teflon plug or "H2 vent" to allow it to H2 leak in case of over charging as Teflon allows tiny hydrogen gas to escape but prevents moisture from getting into a sealed box.
In an open room, it is no sweat for such a tiny battery. If you had a small room full of big batteries , I would be cautious about venting.
You should be able to hear bubbling of H2 in a lead acid battery being created but watch out for ESD static discharges in an enclosed space. H2 gets easier to trigger above the lower explosive limit of 4%. Hydrogen is odorless, colorless and tasteless, so unless you can hear it, you cant see or smell it , but you may smell the sulfur-oxide gas which is often added to H2 tanks or from the sulphuric acid.