A "suspicious" thing makes you feel distrustful. Some examples of "suspicious" things include:

• A friend tells you that he stayed at home last night, but seems like he might be lying.
• There's a suitcase laying on the floor in the middle of a crowded airport, with no one standing next to it.

So an object is "suspicious" if you think it might be dangerous. When you see something and start to think that it's suspicious, you're "noticing something suspicious".