Translation of 'kain po' in specific context

I've noticed something Filipino employees say (only in the work context, haven't observed this in an everyday setting). For example when they are eating in our office pantry and I walk in they say 'kain po'.

I've learned this is more like 'kumakain kami' (FYI) vs 'kumain po kayo' (invitation). Is this correct? What's the English translation of 'kain po' in this context? And where does this custom come from? Genuinely curious!