> a few breaths

I've sometimes gotten a whiff of a toxic fume, and my instant reaction is to hold my breath and leave the area. Not inhale a few breaths.

I'm sure the astronauts are fit enough they could hold their breath long enough to get the helmet on. After all, I expect they probably have a lot of training in getting the helmet on fast. There are many emergencies that would require this, such as rapid decompression, fire, smoke, etc.

I don't know if the astronauts do this or not, but if I was reentering, I'd put the helmet on anyway. It's only for a few minutes.

If it's excess methane, you won't sense it. This is why municipal gas has a smell agent added. The same cannot be done for space fuel. If it's hydrazine, it is about as smelly as gasoline but ammonia, I think, but the toxic dose is below the smell threshold.

So no, you do not get to generalize here.

Then install a methane gas detector which a siren. So yeah, I do get to generalize.