in Romanian that would be ”sunt în serviciul (ciuva)”. This expression „sunt la serviciu”, is only/mainly used to say „I am at work”, or ”at the office” or ”at the job”, or in sport (tennis) „I am at the serve”, „it is my turn to serve the ball”. There is no other meaning for it. ”Serviciu” can mean „service” in an expression like ”Îți fac un serviciu”, (I am doing you a favor/service), but here, not really..

"Ei sunt la serviciul" would be "They are at work". If the sentence has only "sunt" in the beginning then it's refering to the speaker.