It can be a red flag, but it really depends on the context. A coworker acting territorial when you visit your partner at work does not automatically mean something inappropriate is happening. Sometimes people are simply protective of their workspace, stressed, socially awkward, or unsure how to behave in personal situations. However, it becomes concerning if the coworker appears emotionally possessive, tries to exclude you intentionally, makes subtle undermining comments, or seems unusually invested in your partner. The more important factor is how your partner responds. If your partner confidently introduces you, includes you in conversation, and maintains clear boundaries, that’s a healthy sign. But if they seem uncomfortable, avoid acknowledging you properly, or act defensive, that could indicate blurred boundaries. Ultimately, one isolated incident may not mean much, but repeated patterns and how you feel afterward matter. If you leave feeling confused or uneasy, it’s worth paying attention to that instinct and observing the situation over time.

it might or might not
it depends on how your partner reacts
if ( partner )acts bodly, professional and set clear boundaries; you don't need to worry
if partner act suspicious, unprofessional and doesn't set clear office boundaries; acts as if the co- worker is his /hers
then you can consider it a red flag
but before you deem him or her a red flag, make sure all signs are there
https://clarity.fm/precious-stone

It could be a red flag, but not always. Sometimes coworkers act territorial due to workplace closeness.

Focus on your partner’s behavior. If they respect boundaries and make you feel secure, it’s usually not a problem.

If you’re uncomfortable, talk openly with your partner about how you felt.