If you are both retired and seeing an amount on Line 104 as "Other Employment Income, it is likely coming from an amount on one of your T4A - Statement of Pension, Retirement, Annuity and Other Income slips.
If you have an amount in Box 118 - Medical Premium Benefits, or Box 119 - Group Term Life Insurance Premiums, they will show up on Line 104 of your tax return as "Other Employment Income".
Here's some more information from CRA on amounts that may show up on Line 10400
