Yes they can. Your previous employer has to revoke your H1 or notify them about your termination of employment. Generally this process is very slow that it may taje a year or two. My previous employer H1 was revoked after 1 year by USCIS.
If it is very necessary to visit USA, please apply for B1 visitors visa and enter. Dont create unnecessary trouble for yourself, even if not caught now it may affect in future. Always do what is right and legal.