If hired property has not been returned it will only be classed as theft in certain circumstances.
B hires a wallpaper steamer from C for a week and fails to return it on the due date. This does not automatically mean it is theft. B must have somehow treated the steamer as if it was their own property, sold it on or moved from the area and taken the steamer with them and in doing so permanently deprived C of the ownership of the steamer.
In some cases, hired property is returned late or there is some misunderstanding. This is likely to be a breach of contract. It is advisable to make some basic enquiries into the matter (or speak to a solicitor) prior to making a formal complaint to the police, so you can give them as much information about the circumstances as possible. There can be a fine line between a civil dispute and theft.
If you are in any doubt (once you have found out why the goods have not been returned), contact the police or a solicitor who will help to explain the law accordingly. A summary of the offence of theft is below, although it is not intended as a comprehensive explanation:
"Theft occurs when someone dishonestly appropriates (takes possession of or makes use of exclusively for oneself/someone else without permission) some property that does not belong to him or her and treats it as his or her own with the intention of permanently depriving the rightful owner of the property."