I can think of two possibilities for the express 921:
1. After Brock Rd/Kingston Rd, north on Brock Rd, east on Hwy 407, south on Thickson Rd, east on Conlin Rd and continues to Harmony Terminal.
2. After Brock Rd/Kingston Rd, south on Brock Rd, east on Hwy 401, north on Hwy 412, east on Hwy...