Show off is built without the preposition of with the meaning of ‘suspect’.
However, it is possible to find in the press examples such as the following: “It was not detected, so the police presume that they were professional thieves”, “He pointed out that a series of irregularities is presumed so as not to find the others involved” or “In principle, a sudden death was presumed.”
He Panhispanic dictionary of doubts indicates that the verb show off is transitive when it means ‘suspect or suppose’hence the complement appears without a preposition and identifies what is suspected: “Doctors presume that the cause of death was hypothermia.” So, It is not appropriate to accompany the verb of the preposition of with this sense.
Nevertheless, show off also means ‘to show pride in oneself or one’s things’ and the object of pride can be entered with ofas the Dictionary of the Spanish language (“These brands have always boasted of having the safest vehicles”), or without herthe latter construction widespread in countries such as Mexico, Peru, Argentina, among others (“When I had a visitor, I presumed the house”), as indicated by the Dictionary of Americanisms. So the sequence boast it is not wrong By herself, but the meaning changes.
For this reason, in the imprecise examples at the beginning, it would have been preferable to write “It was not detected, for which reason the police presume that they were professional thieves”, “He pointed out that a series of irregularities is presumed so as not to find the others involved” and “In At first, a sudden death was presumed.
➤ dequeísmo, a grammatical incorrectness