That is a red flag according to me. If you don't have anything fishy why would you be defensive and secretive. Now a days people spend a lot of time in office. Maybe there is something even they cannot wrap their head around.

Early red flags people usually ignore are subtle changes in behavior. It’s not about normal workplace friendships those are completely fine but sudden secrecy, defensiveness over simple questions, or emotional distance. If someone becomes overly protective of their phone or reacts with anger instead of reassurance when asked calmly, that’s when it starts to feel off. The biggest sign isn’t one dramatic incident, it’s a consistent small shift like, minimizing your concerns, or making you feel insecure for even asking.

At the end of the day, it’s not overthinking if there are repeated behaviors that create confusion. Healthy relationships make you feel secure and respected, not doubtful or dismissed.

Trust your gut!

Early red flags are rarely dramatic.
They are usually subtle changes that create a quiet sense of discomfort that people try to rationalize or ignore.

For example: communication suddenly becomes less open, someone becomes unusually defensive, or there is a feeling that something is being emotionally withheld.

Many high-performing people especially tend to ignore these signals because they are used to managing problems logically instead of listening to their internal signals.

But intuition often detects patterns long before the mind wants to acknowledge them.

Paying attention to these early signals calmly can prevent much larger emotional conflicts later.

well red flags can come in various forms or ways.
red flags can mean various things to different people
red flags is actually in my own words ' various characters, attitudes, mind set ,things and things again!! that make one uncomfortable, scared, uncertain, suspicious and insecure in a relationship
so if you are feeling any of those listed above, you better address it on time or leave