I’m suprised they get hot if they’re turned off - assume you mean “not playing and in standby mode”? How hot do they get? 
 

Yes, sorry for my English, I meant “not playing and in standby mode”.

It’s just a little bit of heat. You can feel it with your hand by touching the back of the Five. I've never noticed it before. Maybe it's normal.

Thanx.

Maybe leave them in standby overnight with the bags off and see if any spot on the speaker feels warm in the morning. In standby the Fives use ~3W, so there will be some level of heat being produced by the PSU and circuits. In open air it might not be noticable or there might be a warmer spot depending on the internal design and heatsink position/ airflow. A bag over them could prevent any heat escaping which over a long enough period might feel a little warmer than expected.

https://support.sonos.com/en-gb/article/sonos-power-consumption-while-idle

Thanks for the reply. Tomorrow is Sunday and finally there will be some peace in the house. I will leave the Fives uncovered and see. Maybe they have always done it and I had not noticed. I was worried about possible damage to the hardware…

I wouldn't think the bag would be an issue, SoundSkins sells cloth covers for Sonos that are intended to remain on even when playing. Search Amazon and you should find them.

SoundSkins are acoustically transparent and a fabric, so if there is any heat it will just escape.

A plastic bag, depending on grade of plastic, could act as a barrier or even just be having the air inside it warmed due to the weather, sun shining on it, high ambient daytime temperature and cool down slower than the rest of the room rather than the speaker itself causing a noticeable different.

Personally if I was needing to put plastic bags over electronic devices for hours a day, rather than loose fabric dust sheets I’d just power them off anyway or move them to another room, but that is probably more just a me thing.

Thank you Stanley. Yes, I know. But those are made of fabric. My covers are plastic, because I need to protect the speakers from dust. Maybe the fabric lets the air circulate a little more.

SoundSkins are acoustically transparent, not thermally transparent. A common 2 to 5 mil plastic bag is going to have less thermal resistance than most cloth. Maybe a 40 mil industrial bag would be more.

Easy enough to toss a thermometer in with the speaker and see what it reads, compare that to the thermal limits in the speaker's manual.

Ok, the speakers have been on stand by and uncovered for a few hours and indeed on the back you can feel a certain heat. They probably have always done it but since I didn't have to handle them I never noticed.
 

@ Stanley: these are very thin bags, the biodegradable ones you get from shopping. They are super cheap, but I change them every day anyway. I chose them because there is a very fine dust (chalk) in the house and a fabric would not have been good.

If you have an infrared thermometer you could measure temperature.