HMRC on occasions look into your tax affairs. Tax fraud is defined by HMRC as behaviour that illegally deprives the Exchequer of Revenue. Put simply, HMRC will look into your tax affairs if they believe you are not paying the correct level of tax.
You may receive a letter from HMRC suggesting you are under investigation. HMRC may not notify you immediately, however, if they decide to formally investigate you, you may receive a letter from one of it’s departments asking for further information.