Upvote:1
Wait for a few more days, if you do not receive the explanation/refusal notice separately contact them for it.
Sometimes these ECOβs make mistakes, overlook things or maybe your application got lost somewhere in the supply chain and your passport returned without being reviewed.It happens.
Of course if they do give you a reason and you are not satisfied you can follow the procedure outlined in Complaint procedure for visit visa to UK 6 month