since引导时间状语从句时，主句用完成时，而且一般情况是用延续性动词。但这里需要注意的不是go abroad，而是have gone (to)。

have gone to 意思是去了某地（还未回来），以下是英文释义：

Has/have gone to refer to someone who has gone to a place but has not yet returned.

In other words, someone who has gone to Hawaii is still in Hawaii having a good time!

也就是说，have gone to本身就有“停留在那里的状态”的含义，而不仅仅是表示完成了“去了”这个动作。

所以，这句话the policeman on duty told him that since his defeat，the ex-prime minster had gone abroad中用go abroad没有不妥。