There are ANTs that are more "covert" and are harder to recognize, because these ANTs are negative thoughts and feelings that we've had for so long that we just assume that they our a part of us, or it's just how we are, or they are a part of our personality.
The main thing you want to focus on right now is to use a distraction when your feeling this way. If (like you said) you are at home and don't know what triggered your anxiety, just say something to yourself like "I don't want to feel this way...these feelings I'm having are liars, I prefer to head in a more rational direction with my thinking". So, while you're not pinpointing the specific ANT thought you had, you are still addressing the problem and using a distraction to get your mind off the negativity and reducing your anxiety. This takes time and practice, but eventually then you will start to recognize the ANTs responsible for causing you to feel anxious, and you will be able to effectively label them and call them out. So basically, while you don't necessarily have to pressure yourself to try and figure out what negative thought you had, but instead just realize that you are thinking negative and feeling anxious, and use a distraction to counter those thoughts and feelings.
Hope that helps.