I received an alert on a joint checking account. The alert was for cash withdrawals. In researching, it became apparent that one of the account owners is in the construction field and appears to have individuals employed by him. Checks are being written to several individuals and in the memo it says weekending and the date. The checks are then cashed by each of the individuals. The account name or titling does not reference the name of any business. Should I have any concerns.

You have a personal account that's clearly being used for business purposes... Think of this as a test of your bank's willingness and ability to respond when the activity you see does not correspond to the activity you were told to expect; i.e. your due diligence monitoring.

At the time this account was opened, the CDD questionnaire was so vague that it did not include questions to drill down on anticipated account activity. A decision has been made to contact the customer and have them open a business account and of course use our current CDD questionnaire that asks more pointed questions. Though I can't prove that taxes are not being withheld for the employees, should I be concerned that this is a possibility.

Personally I think the fact that you're asking the question, looking for consensus tells me you have suspicions about the activity.....thus your answer.

You said in your post that the individuals cash the checks.

Don't assume that the individuals are employees. They may be sub-contractors and your customer may be issuing 1099s.

I agree. You can't track who is paying taxes and who isn't. Let the IRS figure that out.