The archive of the Livonian Governor General contains a lot of material concerning the surprise Saxon attack in February 1700. Let's start from the very beginning:
On 11 january 1699 Governor General Dahlbergh informed Charles XII that some units from "The Royal Polish Army" had been quartered at Birzai, close to the Courland border. In order to find out more about this Dahlbergh had sent an officer to the area. A month later the Governor General reported that Lt. General Flemming had visited Riga for the purpose of buying various items for his regiment of dragoons. However, Dahlbergh remarked, Flemming appeared to be a better statesman than soldier and had a gift for "intrigues".
Another indication can be found in a letter from Dahlbergh to the commander at Kokenhusen major Haij, dated 9 May 1699. Haij had apprently reported troop movements in Courland. The matter did not appear to be cause for concern, Dahlbergh replied, but it would do no harm if Haij very discreetly made inquiries. However, no spy should be sent.
More worrying signs started to appear towards the end of 1699. On 27 December wrote to Charles XII, telling him that seven Saxon regiments were quartered on the other side of Mitau, while one regiment was at Polangen. A lot of rumors were going around, but Dahlbergh was making preparations for an outbreak of hostilities following a "rupture" in Holstein.