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They used Voldemort's name whilst they were there not realising it was Taboo.

This alerted the Ministry (and so the Death Eater's) to their location.They called Voldemort, 'Voldemort', and, seeing as he was currently Tracing all usage of his name, they were able to find them. They must've been good, undercover Death Eaters, seeing as they were actually able to somewhat blend in as Muggles, as well.because of the taboo curse...the mention of the word voldemort will trigger the curse and thus the death eathers find you They had said the world Voldemort which Voldemort had jinxed. It was a taboo word which meant that whenever someone said Voldemort the Death Eaters where able to find their location and any magic concealment or spell would cancel out right after the word had been said. To hide you'd have to quickly perform the charms quickly but this was very tricky seeing that the Death Eaters appearance to wherever a person was was almost immediate.