Revealing mistake: Near the end of the film, when Picard bends the pole down to stab Shinzon, the pole supposedly goes through him and out the other side, but the prop travels several inches down his waist. A shot later, you can see the prop pushing against Shinzon's clothing, obviously not piercing anything. (01:39:17)
Revealing mistake: In the scene just after Picard, Worf and Data arrive back on the ship with the android parts, while examining B4 Geordi is using a static prop tricorder (The tricorder is soundless and lightless) and when he closes it you hear the plastic prop closing, not the typical clicking tricorder sound.
Answer: Long range sensors can show the general location of specific ships (this is part of the reason Romulans and Klingons use cloaking devices). All Shinzon has to do is find a remote planet close enough to the Enterprise that would cause them to be the most prudent choice to investigate. It's definitely a gamble but not one that is made without calculation.
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