English Translation
I suddenly noticed that my watch is missing.
Tips for word order.
A: 時計がないのに急に気がついた。= I suddenly noticed that my watch is missing.
B: 急に時計がないのに気がついた。= 1) I noticed that my watch suddenly disappeared.; or 2) I suddenly noticed that my watch is missing.
If you mean that you lost the watch long time ago but you’ve just realized it, word order A is better or less vague than B.