I have been quite keen on a co-worker for 1.5yrs now, but over the few months I have not worked as often with her so I only see her at work social outtings. During this time, I have noticed during our shifts together a few signs which I have interpreted as potential interest in me, including frequently looking in my direction and smiling for more than a glance, (even raising her eyebrows in a suggestive manner when walking past) and always going out of her way to say hi and hold conversation, but then again due to my feelings this could just be my favourable interpretation, and it could be just her being nice. She is not the most outspoken or extroverted person, and given my anxiety when having conversations with her, the content of our talk often is generic chit chat rather than playful banter. My question lies herein.

At a recent work outting I overheard her say to someone else that she doesn't have anyone special in her life, and that she misses the fuzzy intimacy of having someone when she's alone late at night, but that she doesn't really care for Valentines day, but when I asked her later on if she was doing anything special for Valentines day, her response was no, and when I asked a follow up question of whether she was looking for someone at the moment she said 'no not really', that she would just 'see what happens'. Based on the limited information provided, which would be the more likely of scenarios? A) she was conveying subliminally that she is not really interested in me i.e. to take the hint, b) she was literally conveying that she just doesn't want any sort of relationship or have any interest in anyone specifically at the moment, or the regrettably less likely c) that she may not want a relationship, but still may have some sort of interest in me but not want to convey such interest as I haven't made my interest explicit as yet?

I know this may be a difficult one to respond to without specifics, but a sub-question may be that if indeed she is either just not actively looking for someone and/or possesses some interest in me, in what ways could I make our interactions/conversations more flirty or playful, given the lax frequency with which we see each other due to no shifts being available?

Sounds like she is not dating or looking but is keeping an open mind.
Ask her out to dinner or something and get to know her

Ok so long story short, not sure whether this female colleague feels anything or not, and don't see her that frequently enough to know, there's been subtle non-verbal indications she may like me i.e. always smiles when around, has looked my way if we have a shift together, has once or twice raised her eye-brows up in down in a come-hither manner, and when she sees me always comes n says hi, makes small talk, but when asked generally about her love life, she says she's not really looking for anyone. Anyway, seen her out twice after a long gap of not seeing her, I've suggested "we should catch up" and based on her reply would have proceeded to ask her number and set up a "date", and her reply was "yeh we should do a work thing" or a "(insert company name) night" - implying a group catch up. My question is, having received that sort of response twice when I've suggested to catch up (albeit I didn't press on and say 'even just you and me'), would it appear that its just a statement of catching up as a group because that's been the only way we've seen each other in a social setting (ie as a group) or is it more likely that she said that because she was reluctant to have a one-on-one catch-up?

The only way you can know is to ask her out.
If you have the time when you see her strike up a conversation.