As far as I know it's an idiomatic expression meaning - as B. Shaw says in you example - Not likely! = It's not likely to happen. / Fat chance! / Only in your dreams!
Of course, 'catch' is kind of a verb of perception in this case followed by an object and a participle (present participle here).
Catch her committing suicide! = You won't see that happening. Why would she want to commit suicide when she had just decided to marry some Mr. Roddy.
(You won't) Catch me doing it! = No way I'll be doing it!
You won't catch me signing any contract! = This is not going to happen, I won't sign it, period.
Hope this helps...
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