Boromir was at Hellsing HQ dealing with the usual bout of things a new job brings: contracts to sign, places to get shown around, so on and so forth.

(All of which is new to him since he's never had a job so much as a duty he was born into.)

Just as he was preparing to leave, he heard the gathering out back and went to see what was going on. He'd thrown his own share of feasts and celebrations for his men post-battle, so while he's not familiar with the term 'barbecue,' it certainly looks familiar.

It's quite possible someone might see him standing at the book door while he looks on for a moment.