Because of my history I have some strong opinions on this. I may need more time to get over it, but here goes...
If you are in a serious relationship, wether it be married or commited to each other. Flirting is disrespectful to your partner.
I have also found it hides a strong possiblilty that they are attracted to the other person, but cloak it a way so you feel you are overreacting if you are uncomfortable with this 'flirtation'. Doesnt necessarily mean anythings going on though.
I just dont think you would even flirt with someone, if you did not have an atrraction on some level.
If it goes on for along time, it has been my experiance, that something more is going on. If longterm flirtation with the same person suddenly stops, thats an even bigger red flag.
I've been there and I've watched it from the side too.
Next time , eye poking will happen.
.-( [that's some one that just got an eye poke, LOL]