Which of the following is a sign of potential financial exploitation in vulnerable populations?

Sudden changes in financial status serve as a critical indicator of potential financial exploitation in vulnerable populations. These changes can manifest in various ways, such as unexpected withdrawals, a sudden inability to pay bills, frequent transactions that seem out of character for the individual, or the unexplained transfer of assets. Such patterns can suggest that someone may be taking advantage of a person's weakened state, often due to age or disability, to manipulate or control their finances without consent.

The other options, while they indicate other forms of potential abuse or neglect, do not specifically point to financial exploitation. Maintaining contact and police presence relate more to safety concerns or potential physical abuse, while forcing someone into sexual acts pertains to sexual exploitation, which involves different dynamics. Therefore, the most direct and relevant sign of financial exploitation is indeed the sudden and unexplained changes in a person's financial circumstances.