A CRB check at 'standard' level will only show convictions, cautions, etc.
However a check at 'enhanced' level will also show anything that a senior police officer (or other relevant authority) believes to be relevant.
The investigation into the Soham murders severely criticised the police for not retaining information relating to allegations which had previously been made against him, and then passing them on when his enhanced CRB check was undertaken. Since then, police forces have retained far more information about people (many of whom will be entirely innocent), and included it in far more CRB checks.
A couple of examples:
(a) I know of a teacher whose former partner was convicted of downloading child pornography. Even though she is no longer in contact with him, his conviction still appears on her CRB check ;
(b) A teacher has posted here, after finding out that his enhanced CRB check showed an allegation of 'grooming' a child for sex. He had never been told about the allegation which had been made against him, neither had he been contacted in any way by the police.
If the police are 100% confident that an allegation made against a person (possibly leading to an arrest, as in your case) they might delete details of that allegation from their files. But if they believe that there might have been just a tiny element of truth behind the allegation, they will almost certainly (following 'Soham') ensure that the information is included in any enhanced CRB check.
It should be noted though, that most employers have no access to enhanced CRB checks. They're only applicable when someone applies to work with (or alongside) children or vulnerable adults, or for work in areas related to law enforcement (such as the police) or national security.
Chris