If your folding bike is not welcome in a building (like a hotel, restaurant, cafe or office), you can hide it in an extra large plastic garbage bag.These plastic bags are for sale at hardware shops and webshops at a low price, and can be carried compactly folded in a saddle bag or coat pocket. These bags also prevent dirt or wet dripping from the bike on the carpet.

Make shure that there is ventilation in the bag, if a wet steel folding bike sits in it for longer than a few hours, to prevent rust.

Instead of a garbage bag, you can also find a bag of thin woven synthetic fabric on the internet, or sew it yourself with a sewing machine. The bag doesn't have to be very thick and sturdy, if you carry the bike by holding it directly on the frame inside the bag, instead of grabbing the bag. You could even attach a shoulder strap directly to the folded bike, inside the bag, by knotting two cord loops to the bike with carbine hooks.

If your folding bike is allowed in a building, you can attach it to a steel construction in the building with a chain lock, if the staff allows such. Like a staircase, or a coat rack post. Or you can do that in a train, when you want to visit the toilet. Then don't cover it in a bag, or people might think it is a bomb.