Maybe it's a typo, but your description says the field is "ClientID" while the syntax uses "ClientNumber". Assuming that's not it, I would test each individual component of the query and confirm that it returns expected results. Select a target folder to find and verify that it is included in each of the result sets. Then start combining the clauses with "&" and confirm that the target folder is still present.

It's a few of things:

• You have an extra backslash in front of the repository name in the path:  {LF:LOOKIN="\Client\ActiveClients\"}
• the search syntax for type is {LF:Name="*", Type="F"} 
• the field name may or may not be correct (ClientID vs ClientNumber)

I was pulled away for some other tasks.

I changed ID to NUM so as not to confuse with the Laserfiche entry ID.

Maruna's reply was the closest to an answer.

I tried this on a different repository but kept receiving an error:

{LF:Name="*",Type=F} & {LF:LOOKIN="Employee-Payroll\INACTIVE_Employees\",  Subfolders=Y} & {[Employee Folder]:[Last Name]="SMITH"}

After looking through other posts, I changed the first ampersand to a dash:

{LF:Name="*",Type=F} - {LF:LOOKIN="Employee-Payroll\INACTIVE_Employees\",  Subfolders=Y} & {[Employee Folder]:[Last Name]="SMITH"}

I was able to find employee folders, but they were not in the location specified in the Search. It returned employee folders from 'Employee-Payroll\ACTIVE_Employees':

When I changed the search to look for in 'Employee-Payroll\ACTIVE_Employees':

{LF:Name="*",Type=F} - {LF:LOOKIN="Employee-Payroll\ACTIVE_Employees\",  Subfolders=Y} & {[Employee Folder]:[Last Name]="SMITH"}

It returned employee folders from 'Employee-Payroll\ACTIVE_Employees':

Correction

{LF:Name="*",Type=F} - {LF:LOOKIN="Employee-Payroll\ACTIVE_Employees\",  Subfolders=Y} & {[Employee Folder]:[Last Name]="SMITH"}

returned employee folders from 'Employee-Payroll\INACTIVE_Employees':