as you are deleting the entry, therefore the displayed result would be unnoticeable (ie: $null)
First of all, thanks DJ_Sol for the clarification of the -s switch in the hash tables.
To RusselB, it shows the result of a deleted item in the hash tables. So it's not a $null.
/hdel -s Test 1
*Deleted item '1' from hash table 'Test'
