Ablative acting as locative in an Inner Mongolian Mandarin topolect

Yuqing Yang, a first-year MA student in our department, was talking to Jingran Joy Luo, another first year MA student in our department, when she noticed something special in Joy's manner of speech.  Namely, Joy used the ablative particle cóng 从 as a locative.  Normally, the locative is indicated by zài 在 in Mandarin.

Joy is from Baotou, which has the largest population (2,650,364 [in 2010]) of any city in Inner / Southern Mongolia.  Joy was totally oblivious to this special usage of hers until Yuqing pointed it out to her.  Although the ablative can be used as the locative in Joy's Baotou Mandarin, a certain criterion has to be met.  That is, there must be an option where the action one is planning will take place.

Here are some examples:

Nǐ shǔjià cóng zhè dāizhe ma?

你暑假从这呆着吗？

"Will you stay here during the summer vacation?" — or will you go back to China?

—

Nǐ cóng nǎ'er chīfàn?

你从哪儿吃饭？

"Where will you eat?" — here, in a restaurant, or somewhere else?

—

Nǐ cóng zhè shuìjiào ma?

你从这睡觉吗

"Will you sleep here?" — or will you go back home or somewhere else?

—

Nǐ bù cóng zhè shuìjiào ma?

你不从这睡觉吗？

"Won't you be sleeping here?" — instead of going home or somewhere else?

—

Wǒmen fàngjià jiù cóng Běijīng dāizhe ba.

我们放假就从北京呆着吧。

"During the vacation, will we just stay in Beijing?" — or will go elsewhere for excursions as well?

The pattern is very clear.  One wonders how it arose?