No. Sympatico may not ask for the modem immediately after you cancel your service. The automated support system is supposed to send you a prepaid courier package for you to return your modem in. Sometimes this system does not work properly and you may not get a box for a very long time or even ever. But somebody may still notice in the future that you did not return the modem. If you fail to return the modem, Sympatico will charge you a penalty of $180.

From the Sympatico Service Agreement:

20. The Service Modem will always remain the property of Your Service Provider. As such, if the Service is cancelled or terminated for any reason, the Service Modem and other hardware delivered to you must be returned by calling 310-SURF to make arrangements to have the Service Modem and other hardware returned to Your Service Provider. We recommend that you return the Service Modem and hardware via Canada Post or courier. You will need to maintain your receipt from Canada Post or the courier as proof of return If the Service Modem is not received within 15 days of your date of cancellation of the Service, a cancellation fee ... of $180.00 will apply.

You can also return the modem to one of the Bell World stores listed here. Ensure that you get a receipt for the return of your modem.